Nip press belt

ABSTRACT

Nip press belt ( 9 ) of a wet press ( 1 ) or a calender with elongated nip, in particular for a paper, cardboard or tissue machine, with a flexible carrier layer ( 9   a ) that is impermeable to liquids and is made of soft rubber having a hardness in the range between  20  and  50  P+J and an integrated textile reinforcement ( 9   b ), and with a covering layer ( 9   c ) that is fixedly connected to the carrier layer or formed together therewith.

This application claims priority under 35 U.S.C. §371 from PCTApplication No. PCT/EP02/10441, filed on Sep. 17, 2002, which claims thebenefit of European Application Serial No. 01 122 208.0 filed on Sep.17, 2001, the disclosures and contents of which are incorporated byreference herein in their entireties.

DESCRIPTION

The invention relates to a nip press belt according to theprecharacterizing clause of Claim 1.

A nip press belt of this kind, in German also called Preβmantel(translatable as “press jacket”), is used in paper, cardboard or tissuemachines to remove most of the water from the product concerned (wetpress) or to finish the surface (calender). Such presses or calendershave an elongated press gap (“nip”) and are therefore also called “nippresses”.

Within the nip the press belt makes sliding contact, by way of its back(lower) surface, with the actual pressing element; therefore this backsurface must have excellent sliding properties. On its front (upper)surface the press belt makes contact either with a pulp felt (by way ofwhich it indirectly contacts the product concerned) or—in the case of acalender—directly with the product, against which it is pressed by arotating roller.

In addition to the above-mentioned sliding properties of the backsurface, another important factor is the impermeability of the belt toliquid, because water should not pass from the product or pulp felt tothe pressing element, nor may lubricant from the latter enter theproduct or the felt. Furthermore, certain elasticity and flexibilitycharacteristics are required.

Nip press belts of various designs are known in the state of the art.

For example, the patent EP 0 420 372 B1 describes a nip press belt ofthis generic kind with a basic web in the form of an endless loopcovered on its inner and outer surfaces with a smooth polymer-resincoating, which makes the web impermeable to liquids and endows it with auniform thickness. The polymer resin that forms the elastomer is herespecified as polyurethane.

The patent DE 50 20 005 C1 discloses another band for use in papermachines, in particular wet presses with elongated nip (nip presses).The band has on its back surface a smooth, flexible band layer that isimpermeable to liquids, and on the front surface there is a carriertread with a fiber layer attached thereto.

The patent DE 42 02 731 A1 also discloses a belt, here termed “jacket”,of this generic kind for a nip press, which comprises an elastomericjacket material and two layers of reinforcing threads. As specificationof the jacket material, reference is made here on one hand to materialcapable of swelling, and on the other hand to polyurethane.

In WO 95/16820 a paper-machine web—specifically a nip press belt—isdescribed in which a carrier web with a polymer coating is provided,which comprises a thixotropic material (for example, aramid or silicaglass). The base material specified here, again, is polyurethane.

Finally, the patent DE 44 38 354 A1 discloses another press jacket madeof elastomeric material, in which is embedded substantially parallel tothe surface a woven layer of an extremely stable material, for examplearamid fibers.

From DE 299 23 825 U1 another press jacket or a press band or a rollercoating for the paper, cellulose, tissue, printing or textile industryis known, which consists of a rubber-polymer and in which are embedded,in in order to increase the modulus of elasticity, natural and/orsynthetic fibers oriented in the direction of travel.

Furthemore, in the applicant's older European patent application 01 109618.7 a soft-rubber nip press belt with a textile reinforcing layer isdescribed.

The objective of the invention is to provide a nip press belt, theworking characteristics of which are improved in comparison to theseknown solutions, in particular with respect to optimized elasticityproperties and advantageous multidimensional bending behavior, so as toobtain quiet running of the machinery combined with low driving powerand a high total running time.

This objective is achieved by a nip press belt with the characteristicsgiven in Claim 1. As a result of the invention there is also provided animproved wet press or an improved calender for the manufacture of paper,cardboard and tissue.

The invention includes the fundamental idea that the nip press belt ismade of an extremely soft elastomer that conforms readily to othersurfaces (and in addition is sufficiently economical), as a result ofwhich the belt as a whole can be endowed with an unusually low bendingmoment and a low overall modulus of elasticity. The invention furtherincludes the idea of constructing a nip press belt based on a substratethat can be used for a large number of applications, namely a unitarycarrier layer made of soft rubber with integrated textile reinforcement,on which (or continuous with which) an elastomeric covering layer orwearing coat is formed. Finally, it is significant for the success ofthe invention that the mechanical properties of the elastomers that formthe carrier layer and the covering layer are not too different from oneanother.

Achievement of the above-mentioned working characteristics, improved inseveral respects, results from the combination of these advantageouslyadjusted parameters.

In addition, the proposed construction offers the opportunity to selectthe material that forms the covering layer from a variety of suitableelastomers, each of which provides special advantages with respect totolerance of changes in temperature, oil or degree of bending, or withrespect to the quality of the belt surface (to allow the manufacture ofparticularly high-quality or specially structured paper surfaces). Thismeasure eliminates practically all previous restrictions with respect tothe surface design. The proposed nip press belt can continue to beemployed in the case of future developments with regard to papermachines—for instance if, as expected, processing temperatures areraised—and can be optimally adjusted for practically any applicationthat will arise.

In a first preferred embodiment the covering layer consistssubstantially of a polyurethane material known per se, the hardness ofwhich here in particular (like that of the material of which the carrierlayer consists) is in the range between 20 and 50 P+J. This PU wearingcoat in particular enables optimization of the surface profile inaccordance with the requirements of the specific application.

In an alternative embodiment, which in particular provides excellenttolerance of changes in temperature, oil and degree of bending, thecovering layer consists substantially of soft rubber, the hardness ofwhich is substantially the same as that of the soft rubber of which thecarrier layer is made, and which can be formed so as to be continuouswith the carrier layer.

In one advantageous embodiment the soft-rubber carrier layer andoptionally provided soft-rubber covering layer of the belt comprise arubber compound that is highly abrasion-resistant, being made of severalrubber or silicone-rubber composites. In particular, these compositesare homogeneously mixed with one another in the compound.

The choice of a specific composition or compositions allows the hardnessof the soft-rubber layer to be adjusted according to the requirements ofthe particular application, i.e. to suit the customer's desires. Thehardness is preferably about 35 P+J.

In order to adjust the breaking strength of the belt to severe demands,a fiber reinforcement or an interlocking material is incorporated intothe elastomer layer. This textile reinforcing component, according tothe information currently available, preferably takes the form ofstrands oriented in the circumferential and/or longitudinal direction ofthe belt. It also seems reasonable to construct it as a nonwoven fabricof staple (short) fibers, either as an independent reinforcing layer orin combination with another type of reinforcement, such as theabove-mentioned strands. The employment of a woven fabric as reinforcinglayer is also possible.

As reinforcing material, in particular modified glass or carbon fiberscan be used, or in particular highly stable plastic fibers. In the lastcase polyester and polyamide copolymers or aramid are the primarycandidates. Depending on the customer's requirements, combinations ormixtures of these materials can usefully be employed.

The textile reinforcement is preferably incorporated near the backsurface of the carrier layer in the elastomer-textile composite, firstin order not to impair the elastic properties of the front surface ofthe belt, and also to ensure a minimal bending moment at the curved partof the pressing element and at the ends of the belt, and thus to achievethe intended overall optimization of the multidimensional curvaturebehavior.

In accordance with the customary specifications of paper machines, thethickness of the carrier layer is advantageously in the range between 4and 6 mm, with a standard value of 5 mm, and the thickness of thecovering layer can vary between about 2 and 6 mm. In view of this, thetotal thickness of the belt (chosen in accordance with the elasticityand stability properties of the individual materials) will in particularbe in the range between 5 and 10 mm, the standard being ca. 7.5 mm.

The front surface of the belt can be made smooth, which will beparticularly appropriate for use in a calender. However, specificallyfor use in a wet press, it can also have a well-defined structure. Thestructure that seems preferable at present consists of recesses in theform of pocket holes. These can in particular be approximatelycylindrical in shape and isolated from one another. However, structuresin which recesses are connected to one another, grooved structures andthe like can also be useful.

Depending on the specific conditions of use, the open cross-sectionalarea of the structured front side (i.e., the total area occupied by therecesses) advantageously amounts to 10–50% of the total area of thefront surface of the belt. For conventional wet-press applications, itseems appropriate for the openings to have an area equal to about 20% ofthe total. In particular, the recesses have lateral dimensions in therange between 0.5 and 5 mm, in particular between 1 and 3 mm. Theirdepth is advantageously in the same range.

According to a further essential aspect of the invention, the nip pressbelt has a stiffness or—converted to take account of crosssection—bending stress distinctly below that of conventional belts. Thusthe force required to achieve a deflection of 15 mm in a three-pointbending test of a specimen 30 mm wide and 5 mm thick, set on supports100 mm apart, is preferably 17 N or less, in particular 13 N or less,and the bending stress is below 110 N/cm² and in particular below 90N/cm². In an embodiment of the belt in accordance with the inventionthat is preferred for practical purposes, a force of 11 N and a bendingstress of 70 N/cm² was measured.

With such elasticity it contributes substantially to a saving of drivingpower and to quiet running of the associated wet press or calender, andthis benefit is not offset by substantial restrictions with respect tothe service life of the belt. On the contrary, the reduction ofdeformation-dependent strain in the material actually has a positiveeffect on the working or service life of the belt.

Other advantages and useful aspects of the invention will be apparentfrom the subordinate claims, as well as from the following descriptionin outline of an exemplary embodiment with reference to the figures,wherein

FIG. 1 is a schematic drawing of the wet-press section of a papermachine in longitudinal section, and

FIG. 2 is a simplified cross-sectional drawing of an embodiment of thenip press belt of such a wet press, constructed in accordance with theinvention.

FIG. 1 shows part of the nip of a wet press 1 (nip press) of a papermachine with elongated nip. Opposite a pressing element 3 is disposed apress-roller 5, and in the gap between these two, namely the nip 7,water is removed from a paper web 13 enclosed between the surface of thepress-roller 5 and a nip press belt 9 that slides along the pressingelement 3, with a pulp felt 11.

The nip press belt 9 must on one hand have elasticity and flexibilitysuch that it conforms as well as possible to the curved surface of thepressing element 3 and exerts an elastic pressure uniformly upon thepaper web 13 (by way of the pulp felt 11). On the other hand, it must besufficiently stable to withstand the high, long-term stress (tension,pressure and vibration) in the nip 7 for a service time that iseconomically acceptable. The hardness of the soft rubber in the standarddesign is 35 P+J, but it can be varied within broad limits, as desiredby the customer, by appropriate variation and mixing of rubbercomposites. The soft rubber can be used in the long term at temperaturesabove 100° C., and can tolerate temperature peaks of up to ca. 130° C.

The total area of the openings amounts to 20% of the area of the frontsurface of the belt or, alternatively, grooves (not shown) withcomparable dimensions can be formed in the covering layer. To thecarrier band 9 a is fixedly connected a polyurethane wearing coat 9 cmade of a PU material, the hardness of which is likewise about 35 P+Jand the mechanical properties of which thus are largely the same asthose of the carrier band.

A nip press belt 9 suitable to meet these demands is shown(schematically) in cross section in FIG. 2. This nip press belt 9 has asits base a carrier band 9 a made of soft rubber into which, near itsback surface, is integrally incorporated a textile reinforcement 9 bconsisting of carbon fibers or modified glass fibers or aramid fibers.The overall thickness of the belt is 7.5 mm; in the region of thelateral lips 9 d it is ca. 5 mm. In the free surface of the wearing coat9 c are formed pocket holes 9 e with a diameter of 2.5 mm and a depth of2 mm. The thickness of the carrier layer or carrier band 9 a is 5 mm,and the thickness of the PU covering layer or wearing coat is 2.5 mm.

The embodiment of the invention is not restricted to this example, butis also possible in many modifications, with respect both to thematerials of which the individual components are made and to thegeometric dimensions.

LIST OF REFERENCE NUMERALS

-   1 Wet press-   3 Pressing element-   5 Press-roller-   7 Nip-   9 Nip press belt-   9 a Carrier band-   9 b Textile reinforcement-   9 c Wearing coat-   9 d Lateral lip-   9 e Pocket hole-   11 Pulp felt-   13 Paper web

1. Nip press belt of a wet press (1) or a calendar with elongated nip,in particular for a paper, cardboard or tissue machine comprising: aflexible carrier layer (9 a) that is impermeable to liquids and is madeof soft rubber having a hardness in the range between 20 and 50 P+J andan integrated textile reinforcement (9 b); and a covering layer (9 c)fixedly connected to the carrier layer or formed together therewith,which consists substantially of soft rubber and has a hardness in therange between 20 and 50 P+J; the belt being characterized in that in athree-point bending test of a specimen of the nip press belt 30 mm wideand 5 mm thick, set onto two supports 100 mm apart, the force that mustbe exerted to produce a deflection of 15 mm is 17 N or less, and theassociated bending stress is 110 N/cm² or less.
 2. Nip press beltaccording to claim 1, characterized in that the carrier layer (9 a) andthe soft-rubber covering layer (9 c) comprise a rubber compoundconsisting of a plurality of rubber and/or silicone-rubber compositesthat are homogeneously mixed.
 3. Nip press belt according to claim 1,characterized in that the textile reinforcement (9 b) comprises modifiedglass fibers, carbon fibers and/or highly stable plastic fibers.
 4. Nippress belt according to claim 1, characterized in that the textilereinforcement (9 b) comprises threads oriented in the circumferentialdirection of the nip press belt.
 5. Nip press belt according to claim 1,characterized in that the textile reinforcement (9 b) comprises anonwoven fabric of staple (short) fibers.
 6. Nip press belt according toclaim 1, characterized by the carrier layer (9 a) and/or covering layer(9 c) having a hardness of ca. 35 P+J.
 7. Nip press belt according toclaim 1, characterized by the covering layer (9 c) having a structuredfront surface with recesses (9 e).
 8. Nip press belt according to claim7, characterized in that the recesses (9 e) have lateral dimensions anda depth in the range between 0.5 and 5 mm.
 9. Nip press belt accordingto claim 8, characterized in that the open cross-sectional area of thestructured front surface accounts for a proportion of between 10 and50%, of the total area of the front surface.
 10. Nip press beltaccording to claim 1, characterized in that the carrier layer (9 a) hasa thickness in the range between 4 and 6 mm, and the covering layer (9c) has a thickness in the range between 2 and 6 mm.
 11. Nip press beltaccording to claim 1, characterized in that in the three-point test aforce of 11 N is to be exerted.
 12. Nip press (1) with an elongated nipof a paper, cardboard or tissue machine, with a nip press belt (9)according to claim
 1. 13. Calender with elongated nip of a paper,cardboard or tissue machine, with a nip press belt according to claim 1.14. A calender with elongated nip of a paper, cardboard or tissuemachine, with a nip press belt comprising: a flexible carrier layer thatis impermeable to liquids and is made of soft rubber having a hardnessin the range between 20 and 50 P+J and an integrated textilereinforcement; and a covering layer fixedly connected to the carrierlayer or formed together therewith, which consists substantially of softrubber and has a hardness in the range between 20 and 50 P+J.